The polycomb-group (Pc-G) genes of Drosophila are required to maintain appropriate repression of genes that regulate developmental processes. Pc-G genes are conserved in mammals, and mutation of the mammalian homologs leads to severe developmental defects. It is believed that Pc-G proteins function, at least in part, by forming complexes that establish a repressive chromatin structure that maintains the 'off' state of homeotic genes. In order to investigate the mechanism of repression, it is necessary to purify Pc-G complexes and to establish assays that display the function of these complexes. This proposal describes an analysis of a Pc-G complex called PRC1 that has been isolated from Drosophila and that contains several Pc-G gene products. This analysis includes the following aims: 1) Identification of the proteins that make up PRC1. There are several unknown proteins in PRC1, and these proteins will be sequenced, their cDNAs will be isolated, and the encoded proteins will be characterized using genetic and biochemical approaches. 2) A detailed functional analysis of PRC1 will be performed. 3) The human homolog of PRC1 will be isolated and characterized. These experiments should increase the knowledge of how genes are maintained in a repressed state during development.